1. Field of the Invention
The present invention relates to a projection lens and an optical surface determination method.
2. Description of Related Art
Conventionally, there has been known a vehicular headlamp configured such that a light-emitting element is placed to face forward in vicinity to a predetermined point on an optical axis extending in a vehicular longitudinal direction so that light from the light-emitting element is directed forward by a light-passing member disposed in front of the light-emitting element (see Japanese Patent Application Publication No. 2005-11704 (JP2005-11704 A)).
The vehicular headlamp is configured such that the light-passing member receives light emitted from the light-emitting element and, after the light is internally reflected by a front face of the light-passing member first and then by a rear face thereof, emits the light from the front face. A central region on the front face of the light-passing member is subjected to mirror finishing so that the light emitted from the light-emitting element is internally reflected. By employing such a configuration, it is possible to realize a thin vehicular headlamp.
In a general refracting optical system or reflecting optical system, if an incoming wavefront and an outgoing wavefront with respect to one refractive surface or reflective surface are defined, it is possible to easily determine a unique refractive surface or reflective surface.
However, in the above light-passing member, light entering from the rear face is internally reflected twice and then emitted from the front face. In view of this, even if an incoming wavefront entering the light-passing member and an outgoing wavefront output from the light-passing member are defined, it is difficult to determine a unique rear face of the light-passing member.